Sustainability and the environmental neutrality are forcing manufacturers to change their production processes. This has lead to an ever increasing use of composite materials as a means (a) to optimize the use of natural resources and (b) as a way to reduce energy consumption. There are two classes of composites, a single isotropic structure designed to have the desired properties all within a single structure, or multiple layers of an isotropic composite in which each layer is designed to perform a specific function i.e., a multi functional composite. Irrespective of the type of composite carbon nanotubes (CNT) are seen as a material that can contribute to providing the desired property for both types of composite.
Carbon nanotubes (CNT or CNTs) are an allotropic form of carbon. There are at least three types of carbon nanotubes; multi-walled (MWCNT or MWCNTs), double walled (DWCNT or DWCNTs) and single walled carbon nanotubes (SWCNT or SWCNTs). SWCNTs have the most desirable physical properties. SWCNTs are among the strongest, most rigid and toughest materials known to man. They are more than 100 times stronger than steel but only ⅙th of its weight, and can conduct electricity and heat better than any other substance. They have interesting aspect ratios (length/diameter), as well as exceptional absorbance capacity, and photo physical chemical behaviors. CNTs are highly sought additives for composites.
Composites usually consist of a fibre and a matrix. The fibre can be petroleum based or, more often in today's drive for sustainability and environmental soundness, the fibres may be derived from a biological source. The matrix is typically a polymer. As is the case with the fibre, biologically derived polymers (or biopolymers) are being seen as a desirable replacement for petroleum derived matrices.
Tai et al. (2008) investigated the reinforcement of phenolic resin with SWCNT. SWCNTs were made by chemical vapor deposition (CVD), and then formed a network of SWCNT bundles. The SWCNT film stripped from the network was mixed with commercial phenolic resin and then sonicated to embed phenolic resin into the SWCNT film. Solvent and moisture were then removed from the SWCNT film. The compounds were then cast in molds at 170° C. and 9.0 MPa for 2 hours to make composites. Post-curing was applied at 200° C. under vacuum for 12 hours. Young's modulus of SWCNTs/phenolic composites with 0.25, 0.50, 0.75, 1.0, 1.50 and 2.0 wt % SWCNTs was increased by 23.6%, 28.6%, 29.7%, 25.5%, 25.1% and 20.7%, respectively (based on pure phenolic resin), and improvement of tensile strength was 11.1%, 16.6%, 16.8%, 18.6 and 20.3%, respectively (based on the pure phenolic resin).
Yan et al. (2009) investigated the grafting of a chemical on the single-walled carbon nanotube (SWCNT) for phenolic resin composites. The SWCNTs were carboxylated in a strong inorganic acid condition with ultrasonication, and then synthesized aldehyde-functionalized SWCNT via two-step reactions, such as acyl chloride formation and its coupling to 2,2-dimethoxyethylamine. The resulting SWCNTs were dispersed in organic solvent and blended with phenolic resin in organic solvent. This mixture was ultrasonicated and then the solvent was removed and all materials (SWCNT and phenolic resin) were kneaded in a rheomixer at 50 rpm at an elevated temperature for 15 minutes. The resulting composite was cured at an elevated temperature for 7 hours under pressure. Yan et al. found increases of tensile strength and modulus for the SWCNT composite when compared with the control.
Mathur et al. (2010) studied the effects of carbon nanotube dispersion on the mechanical properties of phenolic resin composites. They adopted two methods to disperse carbon nanotubes. Firstly, the independently dispersed multi-walled carbon nanotubes and powder phenolic resin (Novolac) in acetone and sonicated for two hours. The two suspensions were then mixed and sonicated for another two hours, after which the mixture was poured into a petri dish and dried at 50° C. for 24 hours. Afterward, a composite was made using conventional hot pressing, and by curing at 180° C. for 2 hours in air. With dry mixing, a powder phenolic resin (Novolac) and MWCNTs were mixed thoroughly in a mixer by ball milling without solvent, and cast into a mold under the same conditions as the wet mixing procedure. Wet mixing showed little improvement in the mechanical properties of MWCNT/phenolic composites, while dry mixing produced an improvement of nearly 160% with 5% (vol.).
The mechanical and structural properties of carbon nanotubes hold potential for a large-scale application in some materials (Endo et al., Potential Applications of Carbon Nanotubes, Chapter 2 in Carbon Nanotubes (Eds. by A. Jorio, G. Dresselhaus, M. S. Dresselhaus, Topics Appl. Physics 111, 13-62, 2008)). Nanotubes are considered to be the ideal form of carbon fiber with superior mechanical properties compared to the best of traditional carbon fibers, where carbon fibers have a specific strength (strength/density) fifty times that of steel and are excellent load-bearing reinforcements in composites (Baughman et al., Carbon nanotubes-the route towards applications, Science, 2002, 297, 787-792). The tensile strength of individual nanotubes may approach several 100 GPa with an elastic modulus in the TPa range, thus nanotubes have mechanical properties exceeding those of traditional carbon fibers.
The nanotubes can sustain large strains under compression. Although nanotubes have a near perfect structure and a high aspect ratio, there are still challenges in adapting these materials for structural applications. There has not been significant progress in developing nanotube-based composites that outperform the best carbon-fiber composites (Endo et al. ibid 2008). Although nanotube-filled polymer composites could have the advantages, the main challenges are 1) creating a strong interface between nanotubes and the polymer matrix due to the atomic smoothness of nanotubes, and 2) dispersing the individual nanotubes in the polymer, not aggregating in multi-walled carbon nanotubes or bundles in single walled carbon nanotubes (nanotubes are easily re-organized into aggregates).
In recent years, attempts have been made to use carbon nanotubes as reinforcing agents for phenolic resins, specifically to act as a matrix. When carbon nanotube were incorporated into a phenolic resin matrix, there were several problems: 1) CNTs needed to be first dissolved or dispersed in an organic solvent; 2) commercial phenolic resins can be in the form of powder instead of liquid; 3) when incorporating CNTs in a phenolic resin, the organic solvent needs to be able to bring both together before it is removed, and the resulting mixture needs to be further mixed by kneading at an elevated temperature; and 4) the resulting CNTs/phenolic resins mixtures were used only as structural composites, but were not suitable to be used as wood adhesives.
Described herein is a method and a manufacturing process that overcomes these problems.